1. Field of the Invention
The present invention relates generally to an instrument preferably employable for a vehicle for indicating a running speed, an engine speed or the like. More particularly, the present invention relates to improvement of a so-called black-out type instrument preferably employable for a vehicle wherein the whole front surface of the instrument is completely darkened when an ignition key is shifted to OFF by a driver.
2. Description of the Related Art
To facilitate understanding of the present invention, a typical conventional instrument of the aforementioned type will be described below with reference to FIG. 3 and FIG. 4.
The instrument generally designated by reference numeral 90 includes a dial illuminating unit 91 for illuminating a dial 93 when an ignition key (not shown) is shifted to ON with a driver's hand and a pointer illuminating unit 92 for illuminating pointers 94 at the same time as the dial illuminating unit 91. The dial 93 and the pointers 94 are covered with a window glass 95 made of a so-called smoked glass plate having suitable light permeability. With this construction, when the ignition key is shifted to OFF, the whole front surface of the instrument 90 is completely blackened. Thus, the dial 93 and the pointers 94 can visually be recognized so as to allow the present calibrations on the dial 93 to be read by him only when the dial illuminating unit 91 and the pointer illuminating unit 92 are illuminated by shifting the ignition key to ON with a driver's hand.
In FIG. 3, reference numeral 96 designates a stepping motor. In response to an output signal from a calculating circuit 97 for rotating the stepping motor 97 in the normal direction or in the reverse direction, the pointer 94 is rotationally driven in the clockwise direction to reach a predetermined position, i.e., a predetermined calibration 93a on the dial 93 corresponding to the present running state of his car detected by a vehicle speed sensor (not shown).
With the conventional instrument 90 constructed in the above-described manner, the dial illuminating unit 91 make it possible to clearly illuminate the dial 93 because the latter is molded of a light permeable synthetic resin. However, since each pointer 94 is prepared such that it serves as light guiding means while its rear surface is coated with a reflective coating film 94a made of a white-colored paint or the like, the pointer illuminating unit 92 operates at a low optical efficiency compared with the dial illuminating unit 91 because of indirect illumination with the light beam reflected from the coating film 94a, resulting in the pointer 94 being unavoidably visually recognized in the darkened state, i.e., with low brightness.
In the circumstances as mentioned above, when the light permeability of the window glass 95 is predetermined, it is required that it is set to a value sufficiently high so as to enable each pointer 94 to be visually recognized. Thus, when the instrument 90 is exposed directly to sunlight while the ignition key is shifted to OFF, the pointers 94, especially, the white-colored coating films 94a are visually observed with driver's eyes through the window glass 95. This leads to a problem that he feels remarkable visual difference from the visual observation at the time of practical use with the ignition key shifted to ON. In practice, it has been requested from users that the foregoing problem should be obviated.